I. Field of the Invention
This invention relates to the production of mineral fibers and to the fibers so produced.
II. Description of the Prior Art
Mineral fibers such as glass fibers, rock wool and ceramic fibers are used in large quantities for industrial and domestic purposes, primarily as heat insulating materials and, in the case of rock wool, for the production of acoustic tiles. The fibers derive their good heat and sound insulating properties from the relatively low conductivity of the fibers themselves and from the fact that their fibrous shapes result in the incorporation of large quantities of dead air spaces in the mass of material. The fibers are also attractive because of their resistance to high temperatures, fire, rot, attack by insects, and the like.
A critical consideration for these materials is their cost. To be attractive to potential users, the materials must be relatively inexpensive otherwise the payback period in terms of energy savings, when they are used as heat insulators, becomes unacceptably long. Mineral fibers are bulky and transportation costs make them expensive if they are transported over long distances; thus the industry generally consists of numerous small scale production facilities supplying their local areas. Such facilities are usually situated close to an inexpensive source of the raw material, e.g. a suitable rock deposit or a source of smelter slag from copper, lead or iron production furnaces.
While mineral fibers may contain a variety of metal oxides or other metal compounds, a main constituent is normally silica which has a relatively low melting point and can thus be easily drawn into fibrous shapes. In the case of glass fibers, silica is the precursor of the fibres (by definition) and therefore the only ingredient. Rock wool normally contains about 6-8 percent of alumina in addition to silica and other ingredients, and ceramic fibers may contain up to about 50 percent, (and sometimes up to 70 percent or more, of alumina with the remainder being mainly silica. It has been observed that the higher the alumina content, the higher is the melting point of the fibers and the more desirable are the physical properties of the fibers. Pure alumina, however, has a very high melting point (above 2200.degree. C.) and is not itself useful for fiber formation. Alumina can of course be added to conventional raw materials in order to increase their alumina contents, but commercial grade alumina and bauxite are expensive. Consequently, alumina is not usually added to the raw materials for rock wool. Ceramic fibers, on the other hand, often use bauxite as a raw material together with glass sand, but the quantity of this material is usually kept as low as possible in order to keep down the cost, or kaolin clay (a mixture of 45-48% alumina and the remainder mainly silica) is used instead. Kaolin, however, usually contains iron and titanium impurities which can act as pollutants and reduce the refractoriness of the product.
There is accordingly a need for an inexpensive source of alumina for mineral fibers which can be used without making the fibers hazardous from the environmental point of view and which is not detrimental to the physical and chemical properties of the fibers.